Of course moonscript would be optional. TIC like PICO-8 rely on LUA to work.
Moonscript as it is more complex than LUA shouldn't be the default choice, I agree. But moonscript is also interresting for TIC (and PICO-8) because it requires less lines to do the same things as LUA. And since the screen size is limited, it would help a lot on big project for advanced programmers.